The best fertilizer for cannabis gives plenty of nitrogen during vegetative growth, then shifts towards potassium and phosphorus once flowering starts, with steady calcium and magnesium throughout. For field-grown hemp, university trials point to a clear range: about 50 lb of nitrogen per acre for fiber, and about 100 to 150 lb per acre for flower and CBD crops.
Cannabis is a hungry plant, but it punishes guesswork. Too little nitrogen early and the plants stay small. Too much late in the season and flowering is delayed, and cannabinoid content can fall. Getting it right is mostly about timing.
One note before we start. Growing cannabis and hemp is regulated almost everywhere, from licensing and THC limits to approved varieties. This guide covers plant nutrition for licensed producers and legal hemp growers. Always follow the rules that apply where you farm.
What cannabis and hemp need
- Nitrogen (N): the main driver of leaf and stem growth in the vegetative stage.
- Phosphorus (P): supports roots early on and flower development later.
- Potassium (K): important for strong stems, water regulation and flower quality.
- Calcium and magnesium: cannabis uses a lot of both. Shortages show up quickly, especially in soilless and hydroponic systems.
- Sulphur and micronutrients: iron, manganese, zinc and boron in small amounts.
Nitrogen rates for field hemp: what the trials say
Field hemp has the best public research, mostly from US university extension programmes.
| Source | Crop type | Nitrogen guidance |
|---|---|---|
| University of Florida (UF/IFAS) | All hemp | Do not exceed 150 lb N per acre; seed and flower may need up to about 200 lb, fiber about 50 lb |
| University of Kentucky (as summarised by UF/IFAS) | Fiber / grain / CBD flower | 50 lb / 100–150 lb / 100–150 lb N per acre |
| Rodale Institute | CBD hemp, US Northeast | Best rate about 150 lb N per acre, possible upper limit 200 lb |
| NC State Extension | Floral hemp | Biomass levelled off at about 125 lb N/acre on clay and 171 lb N/acre on sand |
The pattern is consistent. Fiber crops need modest nitrogen, because too much lowers fiber quality. Flower and CBD crops need more, but most trials found little benefit above about 150 lb per acre. Sandy soils, which lose nitrogen faster, sit at the top of the range.
Split the nitrogen
University of Delaware guidance for fiber hemp is to apply a small share, about 20 to 25%, at planting, and the rest when plants are 12 to 15 inches tall. University of Florida treats split applications as best practice and warns that excess nitrogen can delay flowering and reduce cannabinoids. Splitting also cuts losses to leaching and rain.
Fertilizer by growth stage
The exact numbers depend on the system: field soil, containers or hydroponics. The direction of change is the same everywhere.
| Stage | What the plant needs | Typical fertilizer approach |
|---|---|---|
| Seedling and early growth | Gentle feeding, roots | Low rates; a balanced starter with some phosphorus |
| Vegetative growth | High nitrogen, steady K, Ca and Mg | Nitrogen-rich feeding; most of the season’s N goes in here |
| Transition to flowering | Less nitrogen, more potassium | Reduce nitrogen; increase potassium and phosphorus |
| Flowering | Potassium, phosphorus, Mg and Ca | Low nitrogen; potassium-rich feeding |
| Late flowering | Little new uptake | Feeding reduced as harvest nears |
Choosing the right products
- Field hemp: granular fertilizers spread before planting and as a side-dressing. Common choices are NPK compounds, urea or ammonium nitrate for nitrogen, and potash for potassium. See nitrogen fertilizer and NPK ratios.
- Containers and greenhouses: water-soluble fertilizers, including calcium nitrate, magnesium sulphate and potassium sulphate, allow precise feeding through irrigation.
- Organic systems: compost, manure-based products and approved organic fertilizers. These release nutrients slowly, so plan ahead. Compare in organic vs chemical fertilizers.
Soil pH
Cannabis grows best in slightly acid to neutral soil, roughly pH 6.0 to 7.0, where most nutrients stay available. In soilless and hydroponic systems, growers usually keep the solution a little lower. If the pH drifts out of range, the plant shows deficiency signs even when the nutrients are present.
Signs of nutrient problems
| Problem | Typical sign |
|---|---|
| Nitrogen deficiency | Older, lower leaves turn pale then yellow; slow growth |
| Nitrogen excess | Very dark, glossy leaves; delayed flowering |
| Potassium deficiency | Brown, scorched leaf edges and tips on older leaves |
| Magnesium deficiency | Yellowing between veins on older leaves |
| Calcium deficiency | Brown spots and distorted new growth |
Common mistakes
- Too much nitrogen late in the season. It delays flowering and can lower cannabinoid content.
- Feeding fiber hemp like a flower crop. Excess nitrogen weakens fiber quality.
- Ignoring calcium and magnesium. Especially in soilless systems.
- Chasing symptoms without checking pH. Fix pH first, then reassess.
Fertilizer supply from Agromer
Agromer is a B2B commodity trader. We supply fertilizers in bulk to importers, distributors and large agricultural businesses, including NPK compounds and custom formulas from European factories, urea, potash and DAP. Tell us the grade, quantity, packaging and destination. See our NPK fertilizer and fertilizers pages.
Frequently asked questions
What is the best fertilizer for cannabis?
A nitrogen-rich fertilizer in the vegetative stage, followed by a lower-nitrogen, potassium-rich feed in flowering, with steady calcium and magnesium throughout.
How much nitrogen does hemp need per acre?
University trials suggest about 50 lb N per acre for fiber hemp and about 100 to 150 lb N per acre for flower and CBD hemp. University of Florida advises not exceeding 150 lb N per acre.
What NPK ratio is best for flowering cannabis?
During flowering, cannabis needs less nitrogen and more potassium and phosphorus than in vegetative growth. The exact ratio depends on your growing system and test results.
Is urea good for cannabis?
Urea can supply nitrogen for field hemp, applied in split doses during vegetative growth. It is not suited to late-season feeding, when excess nitrogen delays flowering.
Why are my cannabis leaves turning yellow?
Yellowing of older, lower leaves usually points to nitrogen deficiency. Yellowing between the veins suggests magnesium deficiency. Check the soil or solution pH first, because the wrong pH can lock nutrients out.
Does hemp grown for fiber need less fertilizer?
Yes. Fiber hemp needs much less nitrogen than flower hemp. Excess nitrogen can lower fiber quality.